A nonaqueous battery using a metal ion such as lithium ion or sodium ion as electrolyte basically comprises a positive electrode, a negative electrode and an electrolyte in a battery can having a cap. The cap has an opening for degassing and a sealing metal plate whose circumferential portion is supported by the cap. In more detail, the sealing metal plate is fixed between the upper plate and the lower plate of the cap (the latter is also referred to as an opening sealing plate). In the nonaqueous battery, invasion of water into the battery brings about deterioration of active materials of the electrodes. Therefore, the battery is required to prevent water from invasion.
As a material of the sealing plate (i.e., valve plate or safety valve), a metal foil has been conventionally employed due to its low water permeability. However, the metal foil is poorly welded to the opening sealing plate, etc. and the resultant battery does not show a high air-tightness.
Japanese Utility Model Publication (Gazette) No. 59(1984)-15398 discloses the use of a sealing plate made of a laminate of a metal foil and a polymer film such as polyethylene or polypropylene.
Japanese Patent Provisional Publication No. 62(1987)-259342 discloses the use of a sealing plate made of a laminate of a metal foil and a polymer film of polyolefin in which an unsaturated carboxylic acid such as maleic anhydride is contained in the amount of 3-8 weight %. Examples of the metal foil include an aluminum foil of 20-30 .mu.m, a nickel foil of 5-10 .mu.m and a stainless steel foil of 8-10 .mu.m. Examples of the polyolefin include graft copolymers obtained by grafting a unsaturated carboxylic acid (5 weight %) such as maleic anhydride, acrylic acid, methacrylic acid, itaconic acid or fumaric acid to polyolefin such as polyethylene or polypropylene. The laminate of a metal foil and the graft polymer is, for example, prepared by combining a metal foil and the graft polymer by use of a heated roller.
The use of the sealing plate made of a laminate of a metal foil and the graft polymer improves airtightness of a battery compared with one with a metal foil or a laminate of metal foil and polyethylene or polypropylene.